Electronic safety switch



Nov; 23, 1954 A. R. GRAY ELECTRONIC SAFETY SWITCH File d June 5, 1950 INVENTOR.

ALFRED RAYMOND GRAY Hls ATT Mn" United States Patent ELECTRONIC SAFETY SWITCH- Alfred Raymond Gray, Somerville, Masst, assignor tbyl mesne assignments, .to GuildLe Lacheur, Boston, Mass. Application June 5,1950, Serial No. 166,206. 8 Claims. (Cl..340'-258)fl bridge circuit associated. therewith; and having .-.a..con-. ducting element connectedthereto, said balanced bridge.-

circuit capable of being unbalanced by'the .mere presence or actual contact of the hand of a personapproaching the conducting element, and whereby of the relay is controlled to operatethe. relay'contacts.

Another objectofmy-invention is anpimprovedtelectronic relay having an operating circuit. for -.opening and closing the relay contacts and in vwhichrelay a balanced circuit is associated withthe operating circuit .tocontrol said operating .circuit,,.to open, and: closethe. relay contacts, and in. which. the .closing time of the relay contacts is measurably shorter than the opening time thereof.

A further object of my invention isran improved electronic relay having :an operating circuit. for opening or closing the relay contacts, and in .which a balanced bridge circuit is associated therewith andcontrolsthev said opcratingv circuit and in which-a .conductor,;-connected to said balanced bridge circuit, isv controlled through the agency of the conductor ,by the-:mere presence of the hand or body of a person .touchingor. approaching the.

conductor, to thereby operate the relay contacts.

I am aware that prior to thisinvention variousforms of capacity relays. have been used,-.buti.these devices have been primarily designed to, operate at appreciable distances, and have beensubject .to instability and other.

objectionable characteristics. whichhave made them .un: reliable overappreciable time intervals-.

A principal object of this inventionvis toprovidean.

improved electronic safety switch having 'an operating circuit for opening or closing relay contacts, and-1n which a modified formof balanced bridge.circuit lsflassoclated therewith to control the... said ;.operating-,circuit,a and in which'a conductor or.conductors connecteclto said balanced bridge circuit,'is controlled through, the, agency of.

the conductor. primarily by:actual hand orswitch contact, for example by the hand of a person touching or grasping the conductor, to :thereby operate 'the .:relay.. contacts.

It is likewise a principal object of this invention to provide a simplified form ofv capacity relay'that will remain reliable over appreciable time intervals..,

It is also a purpose of the present. invention-to provide a simplified form of hand capacity relay, a conductor of which may be touched by a person7s hand without any possibility of electrical :shock...

Other objects and'novel features :of the-construction and arrangement of parts. comprisingmy improved relay will appear as the description of. the. invention'progresses.

In the accompanying drawing;

Figure 1 is a schematic view of one :formof .myirnproved capacity relay;

Figure 2 is a schematic view .of another form :of my improved capacity relay.

Referring to the drawing, and particularly to .Figure 1, there is shown a control electrode 1,. thatis wellinsulated from. the ground. The.electrode'1:may be a switch connecting capacitively or conductively toground,

or may be anything desired, as forexample a conductor of any desired length, and located at a point; orarea that.

suchloperating circuit 2,695,402 Patented Nov. 23, 1954 is .to be protected or that is to be the source of control. Good insulation from the ground, of the electrode or conductor 1, is important in order that the special Wheat-- stone bridge shown in Figure 1 may be made high impedance for increased sensitivity in order to reduce the necessaryamplification, .so that resistors7 and 8.may

be about 10 megohms each to eliminate the possibility of shock upon contactwith the control electrode or con-. ductor 1. The values .for resistors 7 and flare chosen.

so that about 20 ,microamperes would flow through .the person approaching or touching thecontrol; electrode or conductor 1 if such person is in good contact with ground.

Such a .smallamount of current. cannot be felt. Ca

pacitor.6, shown adjustable, balances the capacity ofthe control electrode or .conductor 1, or if the control electrode or conductor capacity to ground is sufliciently low,

capacity 6.1nay be omitted. Resistors 2, 3, 4 and .5 should be 10 megohms-eachwhen electron discharge tube.12 is of a type similar to. 6SN7. Resistors 4 and 5 may be omitted, although slightly greater stability is ob-. tained by their use, because of the lowered grid to ground Resistors 2, .3, 4 and. 5 form part of the bridge. These resistors 2, 3, 4 and.5 may be resistance. Wheatstone of the simple carbon or metalized type and the bridge balanced approximately by selection of resistor values-,.

thus reducing its cost to a negligible value.

Transformer 9 is the relays power transformer, ob

taining current for the primary froman alternatingcurrent supply line through the terminals 10 and 11. The

transformer 9 isstandard, except for electrostatic shield 25. The electrostatic shield 25 and the shielded wires.

out-of-phase capacity or leakage coupling, is vested inv the center-tap ground of the secondary. Without this ground, or even with excellent shielding, there is .likely to be a few micromicrofarads capacity and/ or several meg-- ohms leakage between the primary and the secondary.

This capacity or leakage will supply essentially thesame phase to each side of the bridge, across resistors .4 and 5, and agood balance will be impossible. But, withthe transformer ground, anyout-of-phase voltage is shunted to ground through the transformers relatively low impedance. about 325 volts each side .of the center 1tap. Terminals 10 and 11 connect to an alternating current supplyline. Electron discharge tube 12 is a 6SN7. Cathode resistor 14 is 1200 ohms and capacitor 13 (used to prevent de-, generation) is 25 micror'arads. In the simplest form of this embodiment resistor 14 and capacitor 13 may be eliminated.

Plateresistor 15 is 50,000'ohms; coupling capacitor 16 is 0.5 microfarad and diode input resistor 17 is 50,000 ohms. 61-16 (or both halves in parallel)., Diode load resistor- 20 is.l.5 megohms and diode load reservoir capacitor 21 is in the order of 1.00 micror'arad. to ground and B. Terminal 27 connects to a 250 volt filtered power supply. Resistors 23 and 24 form a voltage divider to supply a positive voltage delay bias of about 12.5 volts to the diode input resistor 17, and resistor 23 should be 100,000 ohms and resistor 24 should be 5,000 ohms. In the simplest form, of this embodiment resistors 23 and.24 may be eliminated.

Withtheother circuit values as above specified, relay 28 should be quiet in. operation; should have a coil .re-

sistance. of about 5,000 ohms, should close atabout 4 ma.,

is applied to the grid of the lower section of electrondis-v charge tube 12. 1f resistors 23and 24 are not used, the

Transformer 9 has a secondary voltage of.

Electron discharge diode tube 18 is half of a.

Terminal 20 connects a across resistor 17 produces, after'beingrectified'by'diode 18, a small D. C'. bias voltage across resistor 20 which is supplied as a negative bias to the grid of the upper section of electron discharge tube 12. This bias in turn reduces the normal current in relay 2% from about 9 milliamperes to perhaps 7 ma. In order to have a reliable circuit it is desirable to have the normal relay current, say twice the relay closing current, but from the point of view of economy of current and relay design the normal current should not greatly exceed that ratio. Now, rather than increase the normal current to compensate for the residual signal reduction, the residual effect may be removed by the use of resistors 23 and 24. The voltage delay thus introduced into the diode circuit prevents rectification of the residual signal and consequently the normal relay current is not reduced thereby.

When the hand or body of a person is near, or in contact with, the electrode control, or conductor, several factors operate, singly or together, to produce an unbalance in the bridge circuit, thus contributing to the reliability of operation of the relay.

One effect is the high resistance of the body to ground, 1,000 being sufiicient to unbalance the bridge. In spite of this sensitivity it is easy to obtain reliable operation by mounting the bridge elements and the control electrode on a small insulator, such as polystyrene, with the junctions of 1, 2, 4 and 7, and 3, 5, 6 and 8 similarly placed with respect to ground. With this arrangement, humidity contributes equal leakage to both sides of the bridge and the normal unbalance stays below the voltage delay of diode 18. Another effect is the addition of the body capacity to ground, 50 microfarads being more than sufficient to unbalance the bridge even through a possible 10 megohms of hand resistance, or in series with a like amount of knob-to-hand capacity. Still another effect is the introduction of an out-of-phase hum pick up from the wiring of the room or between the power line andtrue ground, and random noise; anything over 3 volts through megohms is sufiicient to unbalance the bridge. In view of the relatively high voltage supplied to the bridge, and the bridges high impedance, one or more of these said effects is very pronounced under all conditions of operation and humidity.

The bridge unbalance, due to the presence of the hand or body of a person causes a large A. C. signal to appear on the grid of the lower section of the electron discharge tube 12. This signal, after amplification, is applied through coupling elements 15, 16 and 17 to electron discharge diode 18. As this signal is large, compared to the diode voltage delay, rectification takes place and a relatively large D. C.-current flows into capacitor 21.

Before the hand or body of a person approaches control electrode or conductor 1, relay 28 will be drawing about 9 ma. with the charge across capacitor 21 zero. When the hand or body comes very close or in contact with terminal electrode or the coaxial cable, orpair of cables 1, capacitor 21 will rapidly change, and when at about 7 volts, the relay current will have dropped to about 3 ma. and relay 28 will have released. When the charge across capacitor 21'has reached about 15 volts, the re lay current will have dropped to zero, the charge across capacitor 21 will continue to rise and at 5 RC time will stabilize at about volts.

When the hand is-removed, capacitor 21 will discharge slowly and when at about 6 volts, the relay current will have increased to about 4 ma. and relay 28 will have pulled down. When capacitor 21 finally discharges to zero, relay 28 will again be drawing about 9 ma.

Another embodiment of my invention is shown in Figure 2, and in this embodiment the capacity relay and control electrode or conductor 1 is replaced by a device that may be operated by means of a knob or handle. This device may take the form of a rotary commutator, coil, resistor, or capacitor, which generates or modulates a voltage to operate the controlling circuit for the relay.

One such arrangement is shown at the left section of the schematic diagram of Figure 2. Electron discharge tube 38 contains a medium mu triode and a diode. Elements 13, 14, 15, 16, 17, 20, 21, 22, 23, 24, 26, 27, 28, 29, 30 and 31 answer the same description as the corresponding elements shown in Figure 1.

Control arm 32 is secured to and rotated by a conventional knobnot shown. Rotation of the control arm said bridge impedance circuit to produce an .unbalance III the said 4 33 and 34. Rheostat 34 connects to ground, and rheostat 33' connects, through limiting resistor 37, to the B terminal 27. As the control arm 32 is rotated, capacitor 35 is charged and discharged, and a pulsating voltage is applied across resistor 36 to the triode grid of electron discharge tube 38. The remainder of the circuit functions the same as that shown in Figure 1, except that a gas filled grid controlled electron discharge tube 39 is employed to operate relay 28.

This arrangement conserves D. C. power, as relay 28 is supplied with A. C. current from terminal 41.

Resistor 4%} is a grid current limiting device. In the simplest form ofthis embodiment of my invention capacitors 35, 13 and 22, and resistors 14, 40, 23 and 24 may be eliminated. Many other. arrangements of the left section of the schematic diagram of Figure 2 will suggest themselves to those skilled in the art. For example, rheostats 33 and 34 may bereplaced with two variable condenser stators degrees apart, and arm 32 g by a common condenser rotor.

It is to be understood that the left section of Figures 1 and 2 can be interchanged by breaking the connections at point 31.

What I claim is:

1. In an electronic relay; a normally balanced electrical bridge circuit system having two sets of junctions, the input set of junctions being symmetrical and the output set unsymmetrical with respect to ground, and with the said bridge system having at least six impedance elements; a transformed source of relatively high voltage alternating current, including electrostatic shielding and capacitive or conductive center-tap grounding means for essentially eliminating the primary out-of-phase coupling; means for supplying the said current source to the said bridge circuit through two of the said six impedance elements, to effectively provide a very high impedance across both said sets of junctions in spite of the center-tapgrounded voltage source; a control conductor connected to one junction of the said set of input junctions of the said bridge, a second conductor or capacitor connected to the other junction of the said input set, and adapted to maintain the said bridge system normally in balance; an electrically conductive element temporarily associated with the first-mentioned control conductor including a temporary external series impedance circuit provided by the said elements conductance, its contact resistance or capacity to the first-mentioned control conductor, and its conductance or capacity to ground, said temporarily associated element being in the form of a conductive article or the hand 'or body of a person, and adapted to have a very minute current passed from the said bridge through the said temporary external series impedance circuit to produce an unbalance in the said bridge system; switching means; amplifying, rectifying, and integrating means adapted to operate the said switching means when the said bridge system is unbalanced, whereby the said.

switching means is caused to respond to the presence or contact of the said temporarily associated element.

2. In an electronic relay; a normally balanced electrical bridge circuit system having two sets of junctions, the input set of junctions being symmetrical and the output set unsymmetricalwith respect to ground, and with the said bridge system having four ungrounded resistive elements; a transformed source of relatively high voltage alternating current, including means for eliminating the out-of-phase coupling; means for supplying the said current source to the said bridge circuit through two of the said four resistive elements, to effectively provide a very high impedance across both said sets of junctions in spite of the center-tap-grounded voltage source; a control conductor connected to one junction of the said set of input junctions of the said bridge, a second conductor or capacitor connected to the other junction of the said input set, and adapted to maintain the said bridge system normally in balance; an electrically conductive element temporarily associated with the first-mentioned control conductor including a temporary external series impedance circuit provided by the said elements conductance, its contact resistance or capacity to the first-mentioned control conductor, and its conductance or capacity to ground, said temporarily associated element being in the form of a conductive article or the hand or body of a person, and adapted to have a very minute current passed from the through the said temporary external series bridge system; amplifying, recti ying, adapted to operate when the-said bridge system 'isunbalanced by the presence or-contact of the said-temporarily associated element.

3. In an electronic relay; a normally balanced electrical bridgecircuit system having twosets of junctions, the input set of junctions being symmetrical and the output set unsymmetrical with respect to ground, and with the said bridge system having six resistive elements; a transformed source of relatively high voltagealternating current, including electrostatic shielding and capacitive or conductive center-tap grounding means for essentially eliminating the primary out-of-phase coupling; means for supplying the said current source to the said bridge circuit through two of the said six resistive elements, to effectively provide a very high impedance across both said sets of junctions in spite of the center-tap-grounded voltage source; a control conductor connected to one junction of the said set of input junctions of the said bridge, a second conductor or capacitor connected to the other junction of the said input set, and adapted to maintain the said bridge system normally in balance; an electrically conductive element temporarily associated with the firstmentioned control conductor, said element being in the form of a conductive article or the hand or body of a person, and adapted to have a very minute current passed through the said temporary element to produce an unbalace in the said bridge system; switching means; amplifying, rectifying and integrating means adapted to operate the said switching means when the said bridge system is unbalanced, whereby the said switching means is caused to respond to the presence or contact of the said temporarily associated element.

4. In an electronic relay; a normally balanced electrical bridge circuit system having two sets of junctions, the input set of junctions being symmetrical and the output set unsymmetrical with respect to ground and with the said bridge system having at least six impedance elements, four of which are resistive and ungrounded; a source of alternating current; means for supplying the said current source to the said bridge circuit through two of the said six impedance elements, to effectively provide a very high impedance across both said sets of junctions regardless of the current supply conditions; a control conductor connected to one junction of the said set of input junctions of the said bridge, an adjustable capacitor connected to the other junction of the said input set and adapted to allow the said bridge system to be periodically balanced; a conductive element temporarily associated with the first-mentioned control conductor, in the form of a conductive article or hand or body of a person, operable to cause an unbalance in the said bridge system; an electron discharge tube A.-C. amplifier, having its input connected to the said output set of junctions of the said bridge system, and operable from an unbalance of the said bridge system; a rectifier having its input connected to the output of the said A.-C. amplifier, including a voltage-delay bias adapted to reject any small A.-C. signal caused by any residual unbalance of the said bridge system, and including a resistor and a capacitor adapted to integrate a rectified signal caused by a control un balance of the said bridge system, said integration being appreciably faster than the subsequent discharge of said capacitor; an electron discharge tube D.-C. amplifier, or relay tube, having its input connected across the said resistor and capacitor of the said rectifier and having a relay or other switching means in its plate circuit, and adapted to operate the contacts of the said switching means when the integration voltage across its input circuit resulting from an unbalance of the said bridge system has reached a predetermined value, said value being consistent with the characteristics of the aforementioned temporarily associated element, whereby the said switching means is caused to respond to the presence or contact of the said temporarily associated element.

5. In an electronic relay; a normally balance electrical bridge circuit system having two sets of junctions and including preferably a minimum of six resistive elements; a source of alternating current at the power frequency, and including means for reducing the primary out-ofphase coupling; means for supplying the said current source to the said bridge circuit through two of the said six resistive elements; a control conductor connected to one junction of the said set of input junctions of the said bridge, a capacitor connected to the other junction and switching means porarily associated with the said control conductor to produce an unbalance in the said bridge system; switching means; amplifying,,rectifying, and integrating means adapted to operate the said switching means when the said bridgesystem is unbalanced, whereby the said switch-- ing means is caused to respond to the presence or contact of the said temporarily associated element.

6. In an electronic relay; a normally balanced electrical bridge circuit system having two sets of junctions, the input set of junctions being symmetrical and the output set unsymmetrical with respect to ground, and with the said bridge system having four ungrounded impedance elements; a transformed source of relatively high voltage alternating current, including electrostatic shielding and capacitive or conductive center-tap grounding means for essentially eliminating the primary out-of-phase coupling; means for supplying the said current source to the said bridge circuit through two of the said four impedance elements, to effectively provide a very high impedance across both said sets of junctions in spite of the centertap-grounded voltage source; a control conductor con nected to one junction of the said set of input junctions of the said bridge, a second conductor or capacitor connected to the other junction of the said input set, and adapted to maintain the said bridge system normally in balance; an electrically conductive element temporarily associated with the first-mentioned control conductor including a temporary external series impedance circuit provided by the said elements conductance, its contact resistance or capacity to the first-mentioned control conductor, and its conductance or capacity to ground, said temporary associated element being in the form of a conductive article or the hand or body of a person, and adapted to have a very minute current passed from the said bridge through the said temporary external series impedance circuit to produce an unbalance in the said bridge system; an electron discharge tube A.-C. amplifier, having its input connected to the said output set of junctions of the said bridge system, and operable from an unbalance of the said bridge system; a rectifier having its input connected to the output of the said A.-C. amplifier, including a voltage-delay bias adapted to reject any small A.-C. signal caused by any residual unbalance of the said bridge system, and including a resistor and a capacitor adapted to integrate a rectified signal caused by a control unbalance of the said bridge system, said integration being appreciably faster than the subsequent discharge of said capacitor; an electron discharge tube D.-C. amplifier, or relay tube, having its input connected across the said resistor and capacitor of the said rectifier and having a relay or other switching means in its plate circuit, and adapted to operate the contacts of the said switching means when the integration voltage across its input circuit resulting from an unbalance of the said bridge system has reached a predetermined value, said value being consistent with the characteristics of the aforementioned temporarily associated element, whereby the said switching means is caused to respond to the presence or contact of the said temporarily associated element.

7. In an electronic relay; a bridge circuit system having at least four impedance elements; high resistive balanced means for supplying an alternating current voltage to said bridge circuit system; electrostatic shielding means for reducing the out-of-phase capacity coupling from the A.-C. line to the said bridge; at least one control conductor conductively connected to the bridge circuit system to produce a state of unbalance in said bridge upon the approach or contact therewith of a persons hand or body; electron discharge tube amplifying and rectifying means actuated by the unbalance of said bridge system; and electrical switching means operable by said electron discharge tube means.

8. In an electrical system to be actuated by a temporarily associated conductive element; a normally balanced bridge circuit system having three sets of junctions, a voltage source set, an input set, and an output set, and including six resistive elements; a transformed source of alternating current balanced to ground, and adapted essentially to eliminate all -degree currents; at least one control conductor connected to one of the input junctions, and the bridge system adapted to be normally in balance; a hand or other portion of a persons body arranged to be temporarily associated with the said control conductor to produce an unbalance in the said bridge system; switching means; amplifying, rectifying and integrating means adapted to operate the said switch- 5 ing means when the said bridge system is unbalanced, whereby the said switching means is caused to respond to the presence of the said temporarily associated element.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,870,181 Mallory Aug. 2, 1932 15 8 Number Name Date 1,905,332 Barbulesco Apr. 25, 1933 2,312,303 Bierwirth Mar. 2, 1943 2,386,942 Edelman Oct. 16, 1945 2,432,390 Darby Dec. 9, 1947 2,518,380 Rowe et a1 Aug. 8, 1950 OTHER REFERENCES Journal of Scientific Instruments, pp. 103, 104, by

10 Rayner and Willmer, vol. 27, April 195 0. 

